This application claims the priority benefit of Japanese application serial no. 2001-239159 filed on Aug. 7, 2001, 2001-328366 filed on Oct. 25, 2001.
1. Field of Invention
The present invention relates to an apparatus and a method for purifying the air used as a raw material in cryogenic air separation that separates nitrogen and oxygen mainly by distilling the air at low temperatures. More particularly, the present invention relates to an apparatus and a method for purifying the raw air capable of effectively removing nitrogen oxides and/or hydrocarbons from the raw air.
2. Description of Related Art
To produce nitrogen, oxygen and argon, the cryogenic air separation that separates the air by using low temperature distillation is performed. When the raw air is to be supplied for cryogenic air separation, the purification of the raw air is performed in order to remove trace impurities from the raw air. In the purification of the raw air, mainly water (H2O) and carbon dioxide (CO2) are removed. In the cryogenic air separation, nitrogen oxides (e.g., dinitrogen oxide (N2O)) and hydrocarbons having boiling points higher than that of the oxygen are condensed in the liquid oxygen. The nitrogen oxides and the hydrocarbons solidify and deposit in the heat exchanger and the distilling tower at low temperatures, so the heat exchanger and the distilling tower may be jammed. Moreover, these are the causes of possible explosions in oxygen atmosphere and must be prevented in advance.
Therefore, in view of safety, it is highly required to remove nitrogen oxides and hydrocarbons in the purification of the raw air to prevent them from being condensed in liquid oxygen. As a technique for removing nitrogen oxides and hydrocarbons, a method is provided using the adsorbents composed of zeolites to adsorb and remove them.
Japanese Patent Application Laid Open No. 2000-107546 discloses an apparatus that uses an adsorption cylinder to remove H2O, CO2 and N2O. The adsorption cylinder has a laminated structure of a first, a second and a third adsorbing layers that comprise 3 adsorbents corresponding to H2O, CO2 and N2O, respectively. The adsorbent for removing N2O includes, for example, calcium-exchanged X zeolites, sodium mordenite, barium-exchanged zeolites and binderless calcium-exchanged zeolites. Moreover, Japanese Patent Application Laid Open No. 2000-140550 discloses an apparatus that uses adsorbents comprising faujasite zeolites to remove at least a portion of N2O gas from the air. Furthermore, Japanese Patent Application Laid Open No. 2001-129342 discloses an apparatus that uses an adsorbent to remove the nitrogen oxides and the hydrocarbons from the air after the removal of H2O and CO2. The adsorbent is, for example, an X zeolite that has a Si/Al ratio within a range of 0.9xcx9c1.3 and contains calcium and other ions.
However, by using the conventional techniques mentioned above, nitrogen oxides and hydrocarbons, particularly those excluding unsaturated hydrocarbons, are difficult to remove effectively. Therefore, a new technique capable of effectively removing hydrocarbons and nitrogen oxides is desired. Particularly, in view of safety, the development of the method for removing N2O is highly required.
In view of the aforementioned problems, this invention provides an apparatus and a method for purifying the raw air that can effectively remove nitrogen oxides and/or hydrocarbons as used to purify the air used in cryogenic air separation.
The apparatus for purifying the air used in cryogenic air separation comprises an adsorber comprising an adsorption cylinder that has a first adsorbing layer and a second adsorbing layer therein. The first adsorbing layer is composed of a first adsorbent capable of selectively adsorbing H2O in the air. The second adsorbing layer is composed of a second adsorbent capable of selectively adsorbing nitrogen oxides and/or hydrocarbons in the air flowing through the first adsorbing layer, wherein the second adsorbent comprises an X zeolite containing magnesium ion as an ion-exchangeable cation. The second adsorbent containing magnesium is preferably fabricated by exchanging a portion or all of the sodium ions in a sodium X zeolite with magnesium, wherein the magnesium-exchange ratio in total cations is preferably higher than 40%. Moreover, the second adsorbent may comprises an X zeolite contains magnesium and calcium ions as ion-exchangeable cations, wherein the magnesium-exchange ratio in total cations is preferably higher than 5%. Furthermore, an A zeolite containing calcium and magnesium ions as ion-exchangeable cations can be used to replace the X zeolite containing magnesium as the second adsorbent that constitutes the second adsorbing layer, wherein the magnesium-exchange ratio in the total cations of the A zeolite is preferably higher than 5%. In addition, the adsorption cylinder can comprise a third adsorbing layer composed of an adsorbent capable of selectively adsorbing CO2 in the air between the first adsorbing layer and the second adsorbing layer.
The method for purifying the raw air used in cryogenic air separation comprises the following steps. A purifying apparatus is provided comprising an adsorber, which comprises an adsorption cylinder that has a first adsorbing layer and a second adsorbing layer therein. The first adsorbing layer is composed of a first adsorbent capable of selectively adsorbing H2O in the raw air. The second adsorbing layer is composed of a second adsorbent capable of selectively adsorbing nitrogen oxides and/or hydrocarbons in the air flowing through the first adsorbing layer, wherein the second adsorbent comprises an X zeolite containing magnesium ion as an in-exchangeable cation. After the water in the raw air is adsorbed and removed by the first adsorbing layer, the nitrogen oxides and/or the hydrocarbons in the raw air are adsorbed and removed by the second adsorbing layer. The second adsorbing layer may also adsorb and remove CO2. Alternatively, the method can use a purifying apparatus with a third adsorbing layer that is disposed between the first and the second adsorbing layers and is composed of an adsorbent capable of selectively adsorbing CO2 in the air. Thus the method can adsorb and remove CO2 from the air passing the first adsorbing layer by using the third adsorbing layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.